1. [Technical Field of the Invention]
The present invention relates to a radiation discriminative measuring apparatus and a radiation discriminative measuring method for use in industries and research facilities, such as a nuclear industry, a radiology and facilities using radioactivity and capable of discriminating and measuring a radiation from radiations (radioactive rays) in which .alpha. ray, .beta. ray, X ray, .gamma. ray and neutron ray are mixed and enabling a nondestructive test to be performed.
2. [Prior Art]
When radiations penetrate a substance, absorption and scattering vary depending on the type and shape of the substance. If the states of the absorption and scattering are recorded by the photography, a video tape or a digital file, a state of breakage or damage of the substance, change in the substance and a state of charge can be recognized. In such case, a principle similar to that with which the internal state of the human body is diagnosed with X rays is employed. This method of detecting the internal state without breaking an object or a sample required to be measured is called radiography or a nondestructive radiography.
Hitherto, a method using the X ray or the .gamma. ray among various radiations has been known as the radiography. The X ray or the .gamma. ray is able to easily penetrate an object. Moreover, these rays are able to easily penetrate the object if the object has a light weight. Therefore, these rays are widely used to detect the internal state of an object. However, the X ray or they ray easily penetrates an object if the object has light weight. Therefore, the foregoing rays easily penetrate light elements having small atomic numbers. As a result, a substance containing hydrogen or the like concealed in a metal material cannot easily be inspected. Moreover, the X ray or the .gamma. ray cannot easily discriminate a small difference, for example, the difference between boron and carbon which are elements having adjacent atomic numbers.
As an alternative to the X ray or .gamma. ray, radiograph using neutrons has been used. The radiography of the foregoing type is able to discriminate light elements which are contained in a metal object and which cannot be discriminated by the X ray or the .gamma. ray because absorption of neutrons does not depend on the atomic number and neutrons penetrate heavy substances. Each element has inherent absorption and scattering cross sectional area with respect to the neutrons such that neutrons are absorbed by boron in a large quantity. On the other hand, neutrons are not considerably absorbed by carbon. Therefore, nondestructive inspection using neutrons to discriminate light elements has been employed.
At present, radiography using the advantages of both of the radiography using X ray or .gamma. ray and the radiography using neutrons has been employed. Specifically, pyrotechnic product has been nondestructively inspected. The radiography using, the X ray or .gamma. ray and neutrons must perform two times of processes for inspecting one sample by using the X ray or .gamma. ray and neutrons. Therefore, a long measuring time is required and a complicated operation must be performed.
As a method of overcoming the above-mentioned problems, a simultaneous radiography method has been disclosed in, for example, Japanese Patent Laid-Open Publication No. SHO 58-113842, in which californium .sup.252 Cf is employed as a neutron source and a .gamma. ray source. Moreover, a .gamma. ray image detector and a neutron image detector are disposed adjacently so as to simultaneously record images on films set to the detectors. However, the above-mentioned method requires two films for recording images. Therefore, accurate position alignment cannot be performed and a complicated image process must be performed.
A method structured by modifying the above-mentioned simultaneous radiography method has been disclosed in, for example, Japanese Patent Laid-Open Publication No. SHO 61-184444, in which a .gamma. ray image and a neutron image are measured in accordance with the color.
However, the above-mentioned method, having an advantage in that a .gamma. ray image and a neutron image can be measured in accordance with the color, suffers from the following problems.
The simultaneous radiography disclosed in Japanese Patent Laid-Open Publication No. SHO 61-184444 employs combination of a red-light emitting scintillator for a .gamma. ray image and a blue- or green-light emitting scintillator for neutrons. Thus, a .gamma. ray image and a neutron image are measured in accordance with the color.
In actual, the scintillator for the .gamma. ray image has a structure that a fluorescent material emitting red light is applied or evaporated on the surface of a heavy-metal plate. On the other hand, the scintillator for neutrons has a structure that a fluorescent material for emitting blue or green light is mixed or applied to a substance containing lithium (Li-6) or boron (B-10). Neutrons and lithium or boron cause (n, .alpha.) reaction, causing alpha (.alpha.) rays to be generated which develops the blue-light fluorescent material to develop blue color. The blue-light emitting fluorescent member contains a fluorescent material which is zinc sulfide (ZnS:Ag) is employed which is activated with silver.
The method uses one film on which a neutron radiography is recorded in blue, and X ray or .gamma. ray radiography is recorded in red to discriminate the image in accordance with the color. Although the above-mentioned method is able to correct fog caused from the X ray and the .gamma. ray. Moreover, the foregoing method using the fluorescent member in the form of combined with zinc sulfide (ZnS:Ag) activated with silver has an advantage that the amount of fogging with respect to the X ray or the .gamma. ray can be reduced. However, realized sensitivity has been unsatisfactory.
The employed scintillator is not formed of a material through which the .gamma. ray and the neutrons are able to penetrate. Therefore, it was required to employ a structure in which scintillators are disposed in such a manner that a film is interposed between a scintillator for a .gamma. ray image and a scintillator for neutrons. Therefore, it was difficult as a usable technique to perform light emission of three or more colors by disposing three scintillators.
In a case where a film is interposed, since a usual color film has an antihalation layer, a light applied from the rear side of the film cannot accurately be recorded. Therefore, a special color film must be employed, thus causing a problem to arise in that the cost cannot be reduced.
On the other hand, a method for improving the sensitivity to neutrons has been developed. An imaging plate for neutrons has been developed as disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI 4-290985. In comparison with the conventional type using lithium (Li-6) or boron (B-10), the disclosed method has improved sensitivity to neutrons. This method utilizes a stimulation light emission of the fluorescent material which is a phenomenon causing light to be emitted by means of stimulus, such as heat or light, after irradiation with electron rays or radiations. The imaging plate has a structure formed by applying a stimulus fluorescent material. Specifically, the imaging plate uses gadolinium (Gd) in the reactions with neutrons. Moreover, activating material is a sintered material containing praseodymium (Pr), terbium (Tb) or europium (Eu).
The above-mentioned imaging plate has been improved into a developed structure formed by combining an imaging plate for the X ray and an imaging plate for neutrons which is made of lithium (Li-6), boron (B-10) or gadolinium (Gd).
The imaging plate for neutrons has a structure incorporating the stimulus fluorescent material and arranged to capture and store a signal caused from ionizing radiation as a color center. Moreover, a light beam emitted from a reading unit causes fluorescent light to be emitted so as to form an image. Therefore, the described method has advantages that a high sensitivity to neutrons can be realized and an operation in a bright region is permitted. However, there arises a problem in terms of performing a real-time operation because an individual reading operation must be performed after neutrons have been applied. Since the above-mentioned technique has been developed to be adaptable to X ray, high sensitivity to the X ray and that to the .gamma. ray can be realized. Therefore, there arises a problem in that images cannot be distinguished from one another because the X ray and .gamma. ray images cover a neutron image. Furthermore, in this case, neutrons must be shielded and the X ray and .gamma. ray images must be individually be taken so as to process the images. However, the above-mentioned method has not been developed.